First off, a UTI is easily treated with anti-biotics but she will have to see the doctor. Also, the two of you may blame the UTI on the sex which may be likely, but it is in fact, not the only cause of a UTI. Girls for unknown reasons tend to get more bladder infections than guys, and from there a UTI is easily developed. UTIs are developed more in women because germs travel through the urethra into the body in both sexes, and women have much shorter urethra's allowing more germs from the outside world (not during sex necessarily) to enter into the body. That being said, she has to get antibiotics, or it will progress into a possible kidney infection, then things will get a lot worse!
For future reference, tell your girlfriend to do the following to prevent UTIs in the future!:
-Drink water regularly and daily
-Urinate frequently
-Urinate after post-sex every time
About the apology, if I was in your situation, I would level with her for sure! Explain to her that it is the both of your faults , not just yours, you may feel sorry for your part, but do not take 100% of the blame, it is in fact about 50/50 and perhaps 65/35. It's her responsibility post sex to urinate, and the sex the two of you have in a relationship is consensual, so, both of you should know the repercussions of not taking the proper precautions. Afterwards, I would say something to explain to her the precautions to take next time to further prevent this from coming back.
All in all, the overall conversation should be something like:
"Apology --> Reasoning (explain her fault in this too) --> Sympathize (You have your fault in this as well) --> Future measures to take"